getExtension

This accessor method returns a reference to the live list,
not a snapshot. Therefore any modification you make to the
returned list will be present inside the JAXB object.
This is why there is not a set method for the extension property.

For example, to add a new item, do as follows:

getExtension().add(newItem);

Objects of the following type(s) are allowed in the list
ExtensionType